This invention is directed to optical adhesives useful in light transmitting devices, and more particularly to fluorosubstituted monoacrylate based adhesives having low refractive index. The invention is further directed to transfer tapes comprised of at least one layer of a fluorosubstituted monoacrylate based adhesive.
Optical coatings to control light distribution, i.e., anti-glare, anti-iridescence, low reflectance and interference, employ coatings of varying refractive index to obtain the desired light distribution. While fluoropolymers offer low refractive index, generally below 1.4, fluoropolymers typically have poor solvent solubility and poor adhesion to substrates.
For optical applications, fluoropolymers are usually made in situ by radiation curing. Alternatively, fluoropolymers may be extruded as melts. It is desirable, therefore, to provide a solvent-soluble fluoropolymer having low refractive index and good adhesion to substrates.
The present invention is directed to an optical adhesive having a refractive index of less than 1.40 comprising a transparent polymer comprising:
75-100% by weight, based on the total weight of the polymer of at least one fluorosubstituted monoacrylate comonomer of the formula: 
wherein R1 is hydrogen and R2 is a linear or branched fluoroalkyl group having 2 to 20 carbon atoms; and
0-5% by weight, based on the total weight of the polymer of an ethylenically unsaturated comonomer selected from the group consisting of (a) mono- and di-carboxylic acids, (b) hydroxyalkyl monomers, (c) epoxy monomers, (d) carboxylic amides, and (e) N-vinyl lactam monomers. The optical adhesive of the present invention is soluble in organic solvents, and in particular, in non-fluorinated organic solvents. The polymer of the optical adhesive of the present invention has a low glass transition temperature and has the ability to bond well with substrates, including glass substrates and polyethylene terephthalate polyester film substrates typically used in optical devices.
The present invention is further directed to a transfer tape comprising an optical adhesive layer and a carrier layer, wherein the optical adhesive has a refractive index of less than 1.40 and comprises a transparent polymer comprising:
75-100% by weight, based on the total weight of the polymer of at least one fluorosubstituted monoacrylate comonomer of the formula: 
wherein R1 is hydrogen and R2 is a linear or branched fluoroalkyl group having 2 to 20 carbon atoms; and
0-5% by weight, based on the total weight of the polymer of an ethylenically unsaturated comonomer selected from the group consisting of (a) mono- and di-carboxylic acids, (b) hydroxyalkyl monomers, (c) epoxy monomers, (d) carboxylic amides, and (e) N-vinyl lactam monomers.
The fluorosubstituted acrylates of the present invention can be copolymerized to prepare copolymers having specifically desired physical properties, such as refractive index, glass transition temperature, light transmittance and adhesiveness.
The fluorosubstituted acrylate monomers used to form the polymer of the present invention are selected from monoacrylates. Generally, monoacrylates are preferred over monomethacrylates because of their lower refractive indices. Among the monoacrylates that may be used are, for example, those of the formula: 
wherein R1 is hydrogen and R2 is a linear or branched fluoroalkyl group. In one embodiment, the fluoroalkyl group R2 is a fluoroalkyl having 2 to 20 carbon atoms. For example, the fluoroalkyl group may be one of: xe2x80x94CH2CF3, xe2x80x94CH2C2F5, xe2x80x94CH2C3F7, xe2x80x94CH2C4F9, xe2x80x94CH2C5F11, xe2x80x94CH2C7F15, xe2x80x94CH2C8F17, xe2x80x94CH2C9F19, xe2x80x94CH2C10F21, xe2x80x94CH2CH2CF3, xe2x80x94CH2CH2C2F5, xe2x80x94CH2CH2C3F7, xe2x80x94CH2CH2C4F9, xe2x80x94CH2CH2C5F11, xe2x80x94CH2CH2C7F15, xe2x80x94CH2CH2C8F17, xe2x80x94CH2CH2C9F18, xe2x80x94CH2CH2C10F21, xe2x80x94CH2(CF2)2H, xe2x80x94CH2(CF2)4H, xe2x80x94CH2(CF2)6H, xe2x80x94CH2(CF2)8H, xe2x80x94CH2(CF2)10H, xe2x80x94CH(CF3)2, xe2x80x94CH2CF2CHFCF3, xe2x80x94CH2CF2CHF(CF2)6H, xe2x80x94CH2CF(CF3)CHFCF(CF3)2, xe2x80x94CH2C6HF12, xe2x80x94C6HF12, xe2x80x94CH2C10HF20, xe2x80x94CH2C5F9H, 
Particularly useful fluorosubstituted monoacrylates include 1H,1H-heptafluorobutyl acrylate, 1H,1H-pentadecafluorooctyl acrylate, hexafluoroisopropylacrylate, 2,2,2-trifluoroethyl acrylate and 1H,1H,2H,2H-heptadecafluorodecyl acrylate. Another useful fluorosubstituted monoacrylate is a blend of 1H,1H,2H,2H-fluoroalkyl acrylates available from DuPont under the tradename ZONYL(copyright) TA-N.
The fluoropolymer used to make the optical adhesive of the present invention generally contains 75-100% by weight, based on the total weight of the polymer, of fluorosubstituted monoacrylate. In one embodiment, the polymer comprises two fluorosubstituted monoacrylates, wherein the total fluorosubstituted monoacrylate content is within the range of 75-100% by weight, based on the total weight of the polymer.
In another embodiment of the present invention, the polymer comprises 95-100% by weight, based on the total weight of the polymer, of the fluorosubstituted monoacrylate. In yet another embodiment of the present invention, the polymer comprises 99-100% by weight, based on the total weight of the polymer, of the fluorosubstituted monoacrylate.
Fluorosubstituted methacrylates may be substituted for a portion of the fluorosubstituted acrylates described above. Examples of such fluorosubstituted methacrylates include methacrylates of Formula I above, wherein R1 is methyl, or a fluorosubstituted methyl group. Because the fluorosubstituted methacrylates generally have higher refractive index and glass transition temperature than the fluorosubstituted acrylates, only a relatively small amount of the methacrylate is included in the polymer of the optical adhesive of the present invention.
A small amount of fluorosubstituted diacrylate monomer may be added to the fluorosubstituted monoacrylate monomer. The addition of an excessive amount of fluorosubstituted diacrylate, greater than about 1% by weight, causes gelling of the optical adhesive.
In addition to the fluorosubstituted monoacrylate monomer, the adhesive polymer contains at least one ethylenically unsaturated monomer having a polar group. This ethylenically unsaturated monomer enhances the cohesive strength of the adhesive and provides a site for crosslinking. Useful ethylenically unsaturated polar monomers include ethylenically unsaturated mono-, di- and polycarboxylic acids, epoxy monomers, hydroxyalkyl monomers, carboxylic amides, and N-vinyl lactam monomers. The fluoropolymer used to make the optical adhesive of the present invention generally contains up to 5% by weight, based on the total weight of the polymer, of the ethylenically unsaturated polar monomer. In one embodiment, the fluoropolymer contains up to 2% by weight, based on the total weight of the polymer, of the ethylenically unsaturated polar monomer, and in another embodiment, the fluoropolymer contains up to 0.5% by weight, based on the total weight of the polymer, of the ethylenically unsaturated polar monomer.
Useful ethylenically unsaturated mono- and dicarboxylic acids include acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, glutaconic acid, 3-methylglutaconic acid, muconic acid, dihydromuconic acid, methylenemalonic acid, citraconic acid, mesaconic acid, and methyleneglutaric acid. Acrylic acid is particularly useful as the ethylenically unsaturated polar monomer.
Useful ethylenically unsaturated epoxy monomers include glycidyl methacrylate, methylglycidyl methacrylate and allylglycidylether. The ethylenically unsaturated carboxylic amides include N-alkylcarboxylic amides, N-methylol carboxylic amides, and alkylethers of the foregoing amides, for example, acrylamide, methacrylamide, N-methylacrylamide, xcex2-diethylacrylamide, mono-, di- and ester-amides of maleic, fumaric, itaconic and other ethylenically unsaturated dicarboxylic acids, N-methylol acrylamide, N-methylol methacrylamide, and ethers of the foregoing N-methylol amide.
Useful ethylenically unsaturated hydroxyalkyl monomers include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate and hydroxybutyl methacrylate.
Useful N-vinyl lactam monomers include such monomers as N-vinyl pyrrolidone.
In one embodiment, a fluorosubstituted alpha, beta-ethylenically unsaturated dicarboxylic acid may be used. Useful fluorosubstituted alpha,beta-ethylenically unsaturated dicarboxylic acids include bis(1H,1H-pentadecafluorooctyl) fumarate, bis(1H,1H-heptafluorobutyl) fumarate, and mixtures thereof. The fluoropolymer used to make the adhesive of the present invention may contain up to 25% by weight, based on the total weight of the polymer, of the fluorosubstituted alpha,beta-ethylenically unsaturated dicarboxylic acid. U.S. Pat. No. 4,786,658, incorporated by reference herein, describes the use of fumarates in fluorinated polymers.
In one embodiment, a fluoroalkyl ethylene comonomer is polymerized with the fluorosubstituted monoacrylate monomer for the optical polymer. Useful fluoroalkyl ethylenes include perfluorobutyl ethylene, F(CF2CF2)2CH2xe2x95x90CH2.
The fluoroalkyl ethylene, as well as the fluorosubstituted alpha, beta-ethylenically unsaturated dicarboxylic acid described above are particularly useful when the polymer of the present invention is made by a bulk polymerization process. The utility of bis(1H,1H-heptafluorobutyl) fumarate and perfluorobutyl ethylene in bulk polymerization process is that they do not readily homopolymerize, but do copolymerize well with acrylic monomers. The bis(1H,1H-heptafluorobutyl) fumarate and perfluorobutyl ethylene function like a solvent to dissipate the heat of polymerization from the reaction initially. When the initial reactor charge (mixture of monomers and initiator) begins to react, the heat of polymerization must be dissipated to avoid gellation. This is known as the Trommsdorf Effect.
A non-fluorosubstituted monoacrylate monomer having a low glass transition temperature (Tg) may be added to the fluorosubstituted monoacrylate monomer to enhance the adhesive properties of the adhesive. A low Tg monomer, as defined herein, is a monomer wherein its homopolymer has a glass transition temperature of or below 10xc2x0 C. Such monoacrylate monomers include 2-ethylhexyl acrylate, isooctyl acrylate, butyl acrylate, ethyl acrylate, methyl acrylate, and mixtures thereof. The optical adhesive of the present invention may contain up to 5% by weight, based on the total weight of the polymer, of the low Tg non-fluorosubstituted monoacrylate monomer.
The polymer of the present invention has a glass transition temperature (Tg) of at least 10xc2x0 C. below the use temperature. The xe2x80x9cusexe2x80x9d temperature is the temperature at which the adhesive in normally bonded to a substrate. In one embodiment, the polymer has a glass transition temperature of less than 15xc2x0 C., as determined by differential scanning calorimeter (DSC). In another embodiment, the polymer has a glass transition temperature of less than 0xc2x0 C., as determined by differential scanning calorimeter (DSC).
In one embodiment, the polymers are synthesized by conventional free radical techniques in solution, using a solvent such as ethyl acetate. Bulk polymerization, such as that described in U.S. Pat. No. 4,786,552, incorporated by reference herein, may also be used. For monomer systems of low acid content, suspension and emulsion polymerization may also be used. Polymerization of the fluorosubstituted monoacrylates may be initiated by a variety of well known free radical initiators. Useful initiators include compounds such as azobisisobutyronitrile, azobis(2-cyanovaleric acid), and 2,2xe2x80x2-azobis(2-methylbutyronitrile), and the like, and organic peroxides such as cumene hydroperoxide, t-butyl peroxide, t-amyl hydroperoxide, t-butyl perbenzoate, di-tbutyl peroxy phthalate, benzoyl peroxide and lauryl peroxide.
Chemical cross-linkers provided in an amount of up to 2.0% by weight in one embodiment, and in an amount of up to 0.5% by weight in another embodiment, can be used to increase the cohesive strength of the polymer. Aluminum acetyl acetonate (AAA) is a particularly useful chemical crosslinking agent.
In one embodiment, an amorphous fluoropolymer, such as Teflon AF(copyright) commercially available from E. I. duPont de Nemours, is added to the adhesive composition. Teflon AF(copyright) amorphous fluoropolymer has a low refractive index, within the range of 1.29-1.31.
The fluorosubstituted polymer of the adhesive is soluble in an organic solvent, and it may be dissolved in a solvent to obtain a coating composition for application directly to the optical element or onto to a transfer or carrier film or a release liner. A fluorine-containing solvent is not required for adhesive solubility. The solvent used for this purpose includes a ketone such as methyl ethyl ketone or methyl isobutyl ketone, an ester such as ethyl acetate or butyl acetate, an aromatic compound such as toluene or xylene and an aliphatic hydrocarbon compound such as octane or hexane. These solvents may suitably be used in combination. Solvent solubility is indicated by a clear or slightly hazy solution of the polymer in the solvent, with substantially no gel or precipitation.
In one embodiment of the invention, the adhesives are cured by exposure to heat under drying conditions, i.e., the adhesives are cured at elevated temperatures sufficient to evaporate solvent(s) from the composition. Such temperatures typically range from about 70xc2x0 C. to about 120xc2x0 C.
In another embodiment of the invention, the adhesives are radiation cured. Curing of the adhesive compositions of the present invention can be effected by passing the adhesive-coated substrate through radiation equipment that is designed to provide the coated substrate with sufficient residence time to complete the cure of the coating. Curing may be effected in an air atmosphere or in an inert atmosphere such as nitrogen or argon. An inert atmosphere is preferred. The length of exposure necessary to cure the adhesive compositions of the present invention varies with such factors as the particular formulation used, type and wavelength of radiation, dose rate, energy flux, concentration of photoinitiator (when required), the atmosphere and thickness of the coating.
In the present invention, a thickness of from 0.5 xcexcm to 500 xcexcm (dry basis) is sufficient for the adhesive coating. In one embodiment, the thickness of the adhesive is within the range of 5 xcexcm to 300 xcexcm, and in another embodiment, the thickness of the adhesive is within the range of 10 xcexcm to 50 xcexcm.
The optical adhesive of the present invention may be a pressure sensitive adhesive. Alternatively, the optical adhesive may be a heat activated adhesive.
In addition to the adhesive compositions described above, the present invention further provides both a transfer tape and tapes of layered construction, the latter consisting of a core coated on one or both sides with a skin layer comprised of the optical adhesive of the present invention.
Transfer tapes prepared in accordance with the present invention comprise a film of adhesive as described above, and at least one release liner. Thus, the adhesives may be coated on a release liner, such as a silicone or carbamate release coated plastic film or paper. Alternatively, a tape of layered construction can be prepared by coating a core, such as a polyester film, on one or both sides with a xe2x80x9cskin layerxe2x80x9d of fluorosubstituted monoacrylate based pressure-sensitive adhesive of the type described above. The core may be an adhesive of the present invention with a release liner applied to each side of the adhesive to form a xe2x80x9csandwichxe2x80x9d arrangement.
The specific examples presented below will serve to more fully describe how the present invention can be practically used. However, it should be understood that the examples are only illustrative and in no way limit the scope of the present invention.